The invention relates to a piezoelectric precision positioning device for moving an object in three coordinate directions and a method for controlling the precision positioning device.
As precision positioning in the context of the invention, a positioning accuracy of better than 10.sup.-10 m is understood, i.e. a movement in the region of atomic dimensions. Devices of this type form e.g. the basis for the examination of surface topography using the tunnel effect. In this case, the precision positioning device moves a pointed scanning needle at a specified distance in a raster pattern across the surface to be investigated, in which process e.g. the tunnel current is used to regulate the distance and the controlled variable is represented as a function of the position signal of the scanning tip (scanning tunnel microscopy).
The precision positioning device must fulfil two requirements in particular. It must be particularly stable mechanically in order to render the system insensitive to vibrations in the environment. The resonance frequency of the device should be as high as possible in order to be able to achieve as high a scanning raster velocity as possible which, in addition to higher spatial resolution, also makes a better time resolution possible in the observation of dynamic processes.
The devices of this type known hitherto use piezoelectric components as positioning elements. In the case of the piezoelectric tripod described in Phys. Rev. Lett., volume 49, No. 1 (1982), pages 57-61, e.g. three strips of a piezoelectric ceramic situated at right angles to each other are combined in the form of a pyramid. The tip of the pyramid can be moved by altering the length of the strips in the three spatial directions. A disadvantage of this arrangement is its relatively large mass which results in a low resonance frequency and, consequently, in an increased susceptibility in relation to thermal drift phenomena.
In another form of construction known from Rev. Sci. Instrum., vol. 56, No. 8 (August 1985), pages 1573-1576, very small piezoelectric ceramic cubes and metal cubes are glued onto each other in checker-board fashion. Although this positioning device has a smaller mass, a large proportion of the total mass is formed from material which is not piezoelectrically active. The interposed metal cubes also reduce the resonance frequency here.
Common to both forms of construction is the fact that they consist of at least three different pieces of a piezoelectric ceramic which are joined to each other by gluing, screwing, clamping or the like. This introduces mechanical weak points which, under cyclic thermal conditions, are severely loaded mechanically owing to the different thermal coefficients of expansion of the materials which are rigidly joined together.
The invention was therefore based on the object of providing a precision positioning device which exhibits a low vibration sensitivity, has a high resonance frequency which is substantially insensitive to temperature, and, in particular, can also be used in the low-temperature range, and which makes possible a flat constructional form and as simple a construction as possible.